The phone number and mobile number fields are designed to handle both US and Non-US phone numbers.  If you would like to restrict it to be US only, please add the following code to your layouts (or a javascript file in your layouts).

$("[data-validate*='tel']").each(function () {
     var inputElement = document.getElementById($(this)[0].id);
     inputElement.addEventListener('keydown', enforceFormat);
     inputElement.addEventListener('keyup', formatToPhone);
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